It is to be appreciated that any discussion of documents, devices, acts or knowledge in this specification is included to explain the context of the present invention. Further, the discussion throughout this specification comes about due to the realisation of the inventor and/or the identification of certain related art problems by the inventor. Moreover, any discussion of material such as documents, devices, acts or knowledge in this specification is included to explain the context of the invention in terms of the inventor's knowledge and experience and, accordingly, any such discussion should not be taken as an admission that any of the material forms part of the prior art base or the common general knowledge in the relevant art in Australia, or elsewhere, on or before the priority date of the disclosure and claims herein.
Many countries have a significant economic reliance on mining, particularly above ground ‘drill and blast’ mining of quarry rock, ore and coal. In Australia 2,500,000 MT of explosives are used per year for mine blasting. (1 m3 ANFO=0.8 MT; 1 m3 emulsion explosive=1.05 MT). At an approximate cost of AUD$1,000 per MT, in 2013 this represented expenditure of AUD$2.5 billion for Australian mines.
Blasting costs consume a large portion of any mine budget. An average coal mine in the Hunter Valley, Australia will consume AUD$40-50 million per annum of explosives and any savings in terms of cost or efficiency are keenly sought.
Large scale mining in open cut mines requires extensive blasting. As a first step, a large flat area (the blast ‘bench’) is prepared by drilling an array of holes up to 15 meters or more in depth and up to 300 mm or more in diameter. Mining and blast engineers determine the best pattern of holes to drill, the amount and type of explosive to load in each hole, and the sequence in which the holes should be detonated. This determination is based on data including geological information and the size of broken rock required.
The mining industry typically uses two types of explosives (i) bulk explosives comprising emulsion and/or a mixture of ammonium nitrate and fuel oil (ANFO) that can be pumped or loaded into drill holes, and (ii) packaged explosives, which comprise emulsion contained within a plastic ‘sausage’. The ‘sausages’ are of various size to suite the application and may be from 25 to 100 mm diameter and up to a meter or more in length. They are loaded into a blast hole to form a column. ‘Decking’ may also be used to better distribute the energy released by an explosive. Decking typically consists of one or more layers of inert material, water or air, strategically located along the column of explosive.
Wherever possible, ANFO is used because it is the least expensive option compared with bulk emulsion (more expensive) or packaged explosives (most expensive). However, ANFO has limited use in wet locations or wet holes because water ingress causes the ammonium nitrate prill to disintegrate, it degrades explosive performance and generates toxic fumes—particularly NO, NO2 and CO. Many failed, incomplete or poorly performing blasts are due to water ingress into the explosive. Many attempts have been made to remove, or work around the presence of water in blast holes.
In the past, attempts have been made to create dry blast holes by soaking up all the water in the blast hole using super-absorbents, such as cross-linked acrylates or acrylamides. For example, British patent GB-2336863 describes a method wherein a chemical powder sealed in a bag is introduced to water in a blast hole, whereupon the bag dissolves, releasing the superabsorbent chemical powder that reacts with the water to form a gel. Typically the chemical powder is a superabsorbent chemical. These typically include for example, sodium polyacrylate.
Other attempts to counter the effects of water in blast holes have involved modified stemming. Blast holes are often ‘stemmed’ by loading an inert material onto the explosive column to contain the energy released by detonation or minimise the loss of explosive energy out of the collar of the blast hole. Crushed rock is a commonly used stemming material, but it is often inconvenient to have to quarry, crush, transport and load the crushed rock into individual blast holes. Drill cuttings, mud or clay are often used as alternatives.
Commercial stemming alternatives include gelled solutions of nitrate salts, or a formulation that contain nitrate salts, potentially causing nitrate pollution to the muck pile and adjacent ground water systems. As an alternative, Australian patent AU-718409 (corresponding to U.S. Pat. No. 5,585,593) teaches the use of stemming comprising additives such as neutralised acrylic acid polymer or a neutralised mixture of sodium silicate and silicon oxide containing material or a combination of the two. This formulation in bulk or packaged form is nitrate free and thus, avoids contamination of ground water by nitrate salts.
In another approach noted by Zhen-Dong et al (Int. J. Rock Mech. & Mining Sci. 47 (2010)1034-1037), water-silt composite has been used to stem blast holes instead of pure silt. Upon detonation, the blast wave firstly affects the inner wall of the blast hole and tangential and radial tension stresses are produced. When the tension stresses exceed the ultimate tensile strength of the rock, cracks and gaps are created. Simultaneously, a huge load is generated on the inner wall of the blast hole by the reflection of the blast wave in the water. The rock is further deformed and displaced and gaps are created in the inner wall. As the water expands the gap disappears. The energy is transmitted among the rock and the rock is broken. Finally, water vapour with residual pressure issues from the cracks and quenches some of the dust.
One of the problems associated with prior art approaches to counter the effects of water in blast holes is that the additives can only absorb a finite amount of water, and they can diffuse or otherwise be mixed into the explosive column, adversely affecting explosive performance, which is commonly called ‘de-coupling’. This is a particular problem when loaded blast holes ‘sleep’ for an extended period of time. The ‘sleeping’ time is the period between loading and firing a blast hole, and can become extended due to production delays, adverse (lightning activity) weather or bad firing conditions.
Myriad environmental issues are associated with mining, particularly the blasting activities required to break up rock and ore. Some of these environmental issues are also associated with adverse events during detonation of a blast hole and these include:                creation of unwanted fine dust (‘fines’),        noise,        over pressure (air blast),        excessive ground vibration,        rifling (ejection of the explosive column to the atmosphere),        generation of hazards such as nitrous oxide fumes and asbestos fines,        irregular blast performance by the explosive, and        uneven rock break at the toe and along the length of the blast hole.        
The need to control dust at mine sites has led to the development of dust suppression strategies for mine access roads, ore crushers, conveyors, transfer points and stockpiles, but hitherto there has not been an efficient method for dust suppression on a mine bench following a blast. In general, mining activities are halted until bulk dust created from the blast has settled, seriously detracting from mine productivity.
Efforts have been made to address the dust problem by blanketing the mine bench with water, foam or gel immediately prior to or after a blast. However, these have proved unsuccessful due to the difficulty of quickly, efficiently and economically blanketing the enormous area of mine benches, which may cover many square kilometers.
Air and/or water decking has been used to try to improve blasting based on the theory that contained water or contained air are both very efficient at absorbing and transferring energy. For example, attempts have been made to stem blast holes by loading a deck of packaged water on top of the in-hole explosive. The packaging is necessary to avoid water degradation of the explosive, particularly ANFO explosive. As an alternative, air decking, or combinations of air and water decking have been used with mixed results.
Canadian patent application CA-886121 teaches improved suppression of dust and fume generation by stemming blast holes with a gel of high water content containing organic ingredients, a preservative and optionally a wetting agent. The patent teaches the use of a gel produced from cellulose ether, alginate or materials containing methylcellulose, carboxymethylcellulose or from a polyacrylic acid or a derivative thereof.
International patent application WO 02/084206 teaches loading of a blast hole with explosive and a highly absorbent material to absorb water located within the blast hole. Preferred highly absorbent materials include super-absorbent polymers, such as starch graft co-polymers, cross-linked carboxymethyl cellulose derivatives and modified hydrophilic polyacrylates. The highly absorbent material may be loaded into the blast hole in water soluble packages or in free flowing powdered, granular, flake or fibrous form.
International patent application WO 2012/090165 relates to stemming material comprising a super-absorbent polymer and a semi-permeable membrane, soaked with aqueous liquid before or after being loaded in the blast hole, so that it expands into contact with the blast hole walls. The super-absorbent is preferably a polyacrylamide, polyvinyl alcohol, cross-linked polyethylene oxide, polymethylacrylate or a polyacrylate salt.
Accordingly, there is a need for a composition that can be readily modified and tailored to the characteristics of specific blast holes